1) Field of the Invention
The present invention relates to an optical transmitter employed in an optical communications system.
2) Description of the Related Art
In a conventional optical communications system, intensity modulation is commonly carried out by a simple on-off keying method. An intensity modulator enhances not only the transmission speed of each wavelength but also the capacity of transmission by increasing the number of multiplexed wavelengths. As a result, in commercial systems the transmission speed per wavelength has increased to 10 Gbit/s and the number of wavelengths that can multiplexed (hereinafter, “multiplexed wavelengths”) has increased to the tune of a few dozens.
In conventional on-off keying, it is necessary that the interval between two wavelengths be set greater than 2.5 times the bit rate, in order to avoid spectral overlap of two adjoining optical wavelength signals. Consequently, the frequency usage efficiency that is defined by the ratio of the wavelength interval and the signal frequency is limited to 0.4.
As a result, the conventional methods of increasing the transmission speed and the number of multiplexed wavelengths to increase the transmission capacity have their demerits. The transmission capacity cannot be improved unless the band of the transmission channel and the optical amplifier are dramatically broadened.
As a means of solving these problems, study results that relate to improvements in the bandwidth and the frequency usage efficiency of the optical fiber used in the optical transmission channel have been reported. Particularly, improved frequency usage efficiency can enhance the transmission capacity of the existing transmission channel and is cost-effective.
FIG. 9 shows the structure of a conventional optical transmitter (See “0.8 bit/s/Hz of Information Spectral Density by Vestigial Sideband Filtering of 42.66 Gb/s NRZ” W. Idler et al., in proceedings of European Conference on Optical communication 2002, 8.1.5. In this conventional optical transmitter, electrical non-return-to-zero (NRZ) signals that are to be transmitted are first converted to plural optical signals by on-off keying modulation process. The optical signals pass through cyclic filters 23-a and 23-b where the side band on one side of these optical signals are suppressed to vestigial side band (VSB), thereby narrowing the bandwidth occupied by each optical signal. Subsequently, an optical wave combiner 24 combines these optical signals and outputs them as wavelength-multiplexed signals.
As a result, for instance, a frequency usage efficiency to the tune of 0.8 bit/s/Hz can be achieved by wavelength-multiplexing an optical signal of 42.7 Gbit/s at an interval of 50 GHz, as shown in FIG. 10C. Further, in this example, the frequency usage efficiency is defined by obtaining a signal wavelength (40 Gbit/s) after deducting from it an error-correcting redundancy bit.
In the conventional optical amplifier, non-return-to-zero on-off keying format that has a relatively narrow bandwidth is used as a modulation format. Consequently, the signal reception sensitivity is low when compared with that of a return-to-zero (RZ) on-off keying format used in a long-distance communications system.
Although the return-to-zero on-off keying format has the advantage of high signal reception sensitivity, due to the high bandwidth of each optical signal, even after their side band is truncated, the bandwidth can still be up to twice that of the non-return-to-zero on-off keying format.